Take the deepest breath that you can. Depending on who you are (age, sex, and height), that breath ranged from three to six quarts of air and contained millions of very small and largely invisible particles, gases such as carbon monoxide, and a long list of chemicals. Some of those contaminants stayed in your lungs and some were filtered out by the body’s defenses that protect the lungs. Over the course of a day, we take about 12 breaths each minute inhaling and exhaling about 10,000 quarts of air, taking breaths in many different places—home, outdoors, in transit, at work, and in public places—each having their own mix of pollutants. Because we breathe so much air, the dose (the amount inhaled into the lungs) of a pollutant may pose a health risk, even though the concentration is seemingly low. With physical activity we inhale even more air, increasing the quantities of air pollutants reaching the lungs.

Some of the pollutants we inhale are toxic to the lungs and to the body generally, because they move from the lungs into the blood and circulate to the heart, brain, and other organs. Air pollution in outdoor and indoor air is a mixture of many components, including pollens and other biological materials that cause allergic responses, irritants that cause inflammation, cancer-causing agents, and the gas carbon monoxide that interferes with the delivery of oxygen to tissues. Some of the agents have familiar names and are well-known hazards—benzene, formaldehyde, radon, asbestos, and mercury. Nature is an important contributor to the pollutant mixture, adding biological materials, windblown dust, volcanic emissions, and wildfire smoke. Human contributions to outdoor air pollution come mostly from combustion for power generation, heating and cooking, transportation, and industry.

Health Risks

We know that high levels of air pollution can cause dramatically increased numbers of deaths. In the small Pennsylvania town of Donora, a 1948 episode of industrial air pollution was linked to 20 deaths. The London Fog of 1952 was even more dramatic, leading to 10,000 or more premature deaths. At the time of this pollution episode, London was polluted by vehicles, industry, and coal burning to heat homes. Figure 1 shows the steep rise in air pollution (smoke and sulfur dioxide) to levels more than 100 times what we experience in Denver, and the dramatic increase in deaths that followed. These and other pollution-caused epidemics of deaths and illness sparked research that continues today. Over more than a half-century since these episodes, epidemiological research studies in communities and toxicology studies in laboratories have provided findings that link breathing polluted air to increased risk for many health risks, including premature death, heart and lung diseases, lung cancer, worsening of asthma, premature birth and reduced birthweight, reduced lung growth in childhood, and, more recently, accelerated brain aging. This enormous body of evidence on the harms of air pollution is the foundation for air quality control worldwide.

Figure 1. Mortality, Smoke, and SO2 concentrations for Greater London, December 1952¹

Air Quality Indicators

In the United States, air pollution is primarily regulated by the Clean Air Act, first passed in 1963 and amended in 1970, 1977, and 1990 with the expansion of its authorities. Under the Clean Air Act, the Administrator of the Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards (NAAQS) for a set of six pollutants referred to as “criteria pollutants.” These are the major pollutants that are most familiar to the public and are often mentioned when air quality is unhealthy. The most important for health are particulate matter (PM), which is tracked by the level of PM_2.5 (small particles that are inhaled into the deep lung), and ozone, which is the indicator for photochemical pollution. The PM_2.5 comes largely from human activity, particularly the burning of fossil fuels for power generation and industrial purposes, and from tailpipe emissions. Ozone is a secondary pollutant, formed by sunlight-driven chemical reactions that involve nitrogen oxides (emitted by motor vehicles) and organic compounds (also emitted by motor vehicles plus oil and gas wells). The geography and climate of the Front Range make ozone a challenging problem because of the strong sunlight, the barrier of the mountains, and the growing and sprawling population. Under the Clean Air Act, the EPA also regulates another large group of pollutants referred to as “hazardous air pollutants” that come from many different sources. Many pollutants in this group are emitted by specific facilities, referred to as point sources, such as refineries.

The EPA uses the NAAQS to inform the public about the quality of outdoor air, using the Air Quality Index (AQI) which is widely reported with weather information. The AQI is based on the short-term standards and the values reported reflect PM_2.5 and ozone in most places. If the AQI is 100, the measured air pollution level is at the standard, the point separating healthy from unhealthy air. A color scheme provides a guide (Figure 2). Denver is ranked among the nation's "dirty air" cities by the American Lung Association because of the number of alert days that we experience because of high ozone and PM_2.5 levels. 

Figure 2. AQI Basics for Ozone and Particle Pollution

The intent of the Clean Air Act is to protect public health and the environment. For the criteria pollutants, the EPA Administrator is called on to set primary NAAQS “…the attainment and maintenance of which in the judgment of the Administrator, based on such criteria and allowing an adequate margin of safety, are requisite to protect the public health.” The Clean Air Act places a strong mandate on the Administrator to set NAAQS that are protective. To do so, the EPA summarizes all relevant scientific evidence every five years for each of the six criteria pollutants. This periodic review assures that the NAAQS reflects what we know about the risks of air pollution. This process led to the recent reduction in the annual standard for PM_2.5, which was lowered from 12 to 9 micrograms per cubic meter of air. For reference, Denver’s annual average for 2019-2021 was 7.8. 

Air quality control under the Clean Air Act has resulted in much cleaner air (Figure 3). Since 1990, the national average levels for most of the criteria pollutants have declined progressively, except for ozone, which has not dropped over the last decade. Contributors to the decline include emissions controls on vehicles, a shift from coal burning to natural gas and renewable for power generation, and the closure of “smokestack” industry plants. Ozone persists as a challenge because of urban sprawl and continued reliance on petroleum-fueled vehicles. The move to lead-free gasoline led to a drop in atmospheric levels of lead that began in the 1970s and the recent sharp decline in lead (Pb) reflects the closure of several lead-emitting industrial facilities.

Figure 3. Air Quality Trends Show Clean Air Progress

A Global View

Looking at air pollution globally, the picture is mixed. As with the United States, air quality has improved in many of the world’s more prosperous nations, while in many of the low-income countries. Rapid increases in the number of vehicles, many without high-level emissions controls, and coal burning for power generation are the dominant sources in many countries. Much polluting industrial activity moved from high-income countries to low- and middle-income countries. Biomass fuels (wood, animal waste, crop waste, and coal) are widely used for cooking, exposing an estimated 2.3 billion people to smoke indoors; the levels of particles may be very high, particularly if the smoke is unvented. In China and India, which together comprise about a third of the world’s population, air quality worsened over recent decades until the severity of the problem prompted action.  

To capture the scope of the disease burden attributable to air pollution, a comparison is made between the actual number of deaths and the number of deaths that would occur if air pollution were reduced to a theoretical minimum. That comparison indicates an enormous burden of premature death—about 6.7 million per year—from outdoor air pollution and indoor air pollution from biomass fuel burning together. Citing that figure, World Health Organization Director-General Tedros Ghebreyesus labeled air pollution as the "new tobacco". The World Health Organization developed Air Quality Guidelines, which set benchmark targets for the world; the most recent revisions tightened them, making them even more stringent than the US NAAQS. 

Greenhouse gases, the main cause of climate change, are perhaps the most threatening and challenging form of air pollution. The capacity of the atmosphere to dilute emissions of carbon dioxide, methane, and other greenhouse gases has been exceeded. Global warming, with its many consequences, is the result. Rising temperatures may worsen air pollution; higher temperatures increase ozone production and power generation for air conditioning will also worsen air pollution, absent a shift to renewable sources. Reducing greenhouse gas emissions will reduce ambient air pollution as we move from fossil fuels to renewables and conserve more energy. 

Looking to the Future

We are at a pivotal moment in air quality control. The scientific evidence on air pollution and health indicates that even the current levels in the United States threaten health, and in many low- and middle-income countries air pollution levels are well above the US NAAQS and the WHO Guidelines. Some communities are at higher risk because of polluting industries abutting their neighborhoods, and too often these are low-income and marginalized communities. In addition, the consequences of climate change are looming. 

We know what major steps need to be taken: emissions reduction from power generation and motor vehicles, changing the energy mix towards more renewable sources, and sustained reduction of greenhouse gas emissions. There are beneficial and synergistic consequences of measures to control air pollution and measures to reduce greenhouse gas emissions. The challenge lies in truly implementing what we should do—everywhere.

Go deeper: Check out the Clearing the Air podcast on the state of air in Colorado. And read the companion article, Denver’s Air Pollution: Is it a Health Threat We Can Control? by Jon Samet.

Resources

American Lung Association’s State of the Air Report 2023

• Key Findings
• Colorado’s Report Card
• State of the Air: Colorado Press Release
• Health Impact of Air Pollution

EPA AQI Basic Information

• Additional AQI Visualization
• Air Quality Guide for Ozone
• Air Quality Guide for Particulate Pollution

Additional References

1. Brimblecombe P. The Big Smoke: A History of Air Pollution in London Since Medieval Times. First ed. Routledge Kegan & Paul; 1987:220